(a) Field of the Invention
The present invention relates to a method and apparatus for treating exhaust gas produced from the combustion of fuels, and more particularly, to such a method and apparatus wherein the reduction of pollutants is achieved by the use of ozone and water vapor.
(b) Description of the Related Art
Internal combustion engines, which operate by the controlled combustion of hydrocarbon fuels, produce exhaust gases containing the complete combustion products of CO.sub.2 and H.sub.2 O, and also pollutants from incomplete combustion, such as unburnt hydrocarbons (HC) as well as CO, which is a direct poison to human life. Further, due to the very high temperatures produced by the burning of hydrocarbon fuels, thermal fixation of nitrogen from the air results in the detrimental formation of nitric oxides (NO.sub.x).
The quantity of pollutants varies with many operating conditions of the engine, but it is influenced predominantly by the air-to-fuel ratio in the combustion chamber. Conditions conducive to reducing carbon monoxide and unburnt hydrocarbons (a fuel mixture just lean of stoichiometric as well as high combustion temperatures) cause an increased formation of NO.sub.x, and conditions conducive to reducing the formation of NO.sub.x (rich fuel mixture and low combustion temperatures) cause an increase in carbon monoxide and unburnt hydrocarbons in the exhaust gases of the engine. As a result, within the region of stable operation of the internal combustion engine, a significant amount of CO, HC and NO.sub.x is emitted into the atmosphere.
There has been a proposed three-way catalysis (TWC) technology to reduce the pollutants of the exhaust gases. The TWC technology comprises the use of precious metals Pt, Rh, and Ce. The basic operation of Pt is to oxidize CO and HC to CO.sub.2 and H.sub.2 O, Rh is primarily responsible for the reduction of NO.sub.x, and Ce functions as an emitter of oxygen. The activity of Rh, however, is reduced in a high oxidizing atmosphere, and Rh may react with CeO.sub.2 at high temperature, reducing the activity of both.
Reduction technology has also proposed the use of ammonia as a reducing agent and V.sub.2 O.sub.5 --TiO.sub.2 as a catalyst, but this is likely to exhaust toxic ammonia gas according to the concentration of NO.sub.x in the exhaust gas.
Another reducing agent has been proposed using hydrocarbons instead of ammonia in Japanese Laid-Open Patent Application No. 63-100919. A catalyst is used in combination with copper and a porous carrier such as alumina, silica or zeolite. However, this method has drawbacks in that the catalyst is likely to be degraded by SO.sub.x.